Self-contained or closed loop appliances using steam heat for commercial cooking are known. Though such appliances can be nominally self-contained or closed loop, the intrusion of non-condensable gases, such as nitrogen and oxygen, can inhibit the transfer of heat by steam to a cooking surface, causing undesirable “cold spots” or “uneven” temperature zones. Such intrusion of non-condensable gases can occur during the manufacturing process or in use in the field.